In our previous analysis of amino acid sequences of light and heavy chains of immunoglobulins, short hypervariable segments in the variable regions were identified and hypothesized to constitute the complementarity determining residues for antibody combining sites. We then devised an empirical method which attempted to predict three dimensional structures of polypeptide backbones of proteins with a number of known homologous amino acid sequences in terms of phi, psi angles. The tripeptides in these proteins were compared with similar tripeptides in reference proteins whose phi, psi values have been determined by x-ray diffraction studies. This method was applied to the variable region of human kappa light chains, cytochromes c, and neurohypophyseal hormones. In the case of horse oxycytochrome c, phi, psi values have been calculated from experimental data. About 85% of our predicted values were in good or intermediate agreement. Thus, we wish to extend our analysis to attempt to predict polypeptide backbone structures of other proteins for which a number of homologous sequences are available, and try to improve our method when phi, psi values of these proteins are obtained from experimental measurements.